In communication systems that use orthogonal frequency division multiple access (OFDMA), single-carrier OFDMA, or SC-frequency division multiple access (FDMA), UE uplink (UL) transmission power is typically set according to some criteria that allows the transmission to be received with sufficient power at an eNB or other communication node (AP) or node without causing excessive interference to neighboring communications. Limiting UE transmit power also reduces UE power consumption thereby increasing battery life.
One method of controlling UE transmission power is to set the transmit power, PT, according to the sum of an open loop transmit power POPEN and a closed-loop correction term, fCLOSED, subject to the constraint that a UE can transmit at most Pmax:PT=max(Pmax,POPEN+fCLOSED).  Eqn. (1)
In this scheme the open-loop transmit power is equal to a target received power, P0, plus a term that is proportional to an estimate of the DL path loss between the node and the UE, PLc,POPEN=P0+α·PLc(dBm).  Eqn. (2)In this expression P0 is expressed in dBm and PLc is in dB. The proportionality factor, α, termed the fractional path loss compensation factor, satisfies 0≦α≦1. When this factor is less than one, UE's farther from the received node, i.e. where the path loss is greatest and transmit power is greatest, are received with power less than those UE's with lower path loss. Reducing transmit power for the highest power UEs in this way limits inter-cell interference and ultimately increases system capacity. The target received power, P0, is typically signaled to the UE over a DL control channel. The UE's estimate of the DL path loss can be obtained by measuring the average received power of a DL reference signal transmitted by the node to which UL transmissions will be directed and the reference signal's transmission power to obtain the estimated DL path loss estimate, PLc. Using this value along with the signaled value of P0 and α, which is also signaled on a DL control channel, allows the open loop transmit power, POPEN, to be obtained.
The total UE transmit power is the open-loop transmit power P0 plus the closed-loop term fCLOSED expressed in dB. The closed-loop term is signaled by the network using power control commands on a dynamic basis and can be used to fine-tune the total transmit power, for example to compensate for fast fading or to temporarily reduce inter-cell interference.
The power control procedure described above applies when a UE is transmitting to a single node in the network. Improved system capacity especially in terms of cell edge throughput can be achieved with cooperative multipoint reception (CoMP). In CoMP reception, a UE is received by not only a single node but simultaneously by multiple nodes. The nodes are typically geographically separated. Simultaneous reception at more than one node, termed joint reception, is particularly advantageous in heterogeneous deployments where different types of nodes are present in the network. For example, a network may include both macro and low power nodes (LPNs). Macro nodes, also referred to as eNodeBs, have a higher transmit power in the DL than LPNs and consequently a larger coverage area. LPNs on the other hand have a smaller coverage area but are useful for providing hot-spot coverage in areas of high user density. LPNs may also be connected to macro nodes through a high speed link, such as a fiber link. LPNs connected in this fashion may be referred to as remote radio heads (RRHs) or remote radio units. In such a system a UE's UL transmission may therefore simultaneously be received by a macro node plus one or more low power nodes. Decoding UE transmissions is performed by a reception processor connected to the macro node and LPN or RRH. Alternatively, the reception processor could be located within the macro node itself.
UL power control is also required for multiple reception nodes to control inter-cell interference and reduce UE power consumption. The power control procedure described above for the single reception node case, however, cannot be used because the determination of an open loop transmit power requires an estimation of DL path loss between the UE and a single receiving node. However, in the presence of multiple receiving nodes (which may be of different types), the ability of a reception processor to successfully decode a UE transmission signal is a function of the power received at all of the reception nodes. Open-loop power control is therefore a function of not only a single path loss but of multiple path losses.
Another case where open-loop power control is a function of multiple path losses occurs when a UE interferes with one or more nodes, called victim nodes, which receive transmissions from other UEs in the system. This is particularly problematic when propagation delay between the interfering UE and the victim node is large enough that the propagation delay plus any multipath delay spread exceeds the system's cyclic prefix (CP). In OFDMA and SC-FDMA systems, a cyclic prefix which corresponds to an end portion of a block of the FFT-modulated signal is repeated at the beginning of the block. This operation suppresses intersymbol interference as long as the delay in the channel is less than the length of the CP. When the channel between a UE and a victim node is such that the sum of the propagation delay and multipath delay spread is larger than the CP, intersymbol interference occurs and demodulation performance at the victim node is impacted adversely. This problem can be lessened by appropriately setting the transmission power of the interfering UE such that the interference received at the victim node is not too high. Given the propagation losses between a UE and both its desired reception point(s) and victim reception point(s), the problem is therefore to determine a transmit power that allows sufficient received powers at the desired UE reception point while not causing excessive interference at the victim nodes. Both in the case of a UE transmitting to multiple nodes and the case of a UE transmitting to one or more nodes but causing interference at other node(s) there is a need for power control based on path losses to more than a single node. Accordingly, a need exist for a power control procedures which adjust transmit power based on path losses to multiple nodes.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.